Method of producing a synthetic diamond

ABSTRACT

A method of producing a synthetic diamond is disclosed, the method comprising: (a) capturing carbon dioxide from the atmosphere; (b) conducting electrolysis of water to provide hydrogen; (c) reacting the carbon dioxide obtained from step (a) with the hydrogen obtained from step (b) to produce methane; and (d) using the hydrogen obtained from step (b) and the methane obtained from step (c) to produce a synthetic diamond by chemical vapor deposition (CVD).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. § 119 of UnitedKingdom Patent Application No. GB1501992.0, filed on Feb. 6, 2015 whichis hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

Aspects and embodiments relate to a method of producing a syntheticdiamond. In particular, certain embodiments relate to a method of usingcarbon dioxide from direct air capture (DAC) and renewable energy toproduce a synthetic diamond.

BACKGROUND

Carbon dioxide in the atmosphere accounts for a large proportion of the“enhanced greenhouse effect.” Carbon dioxide concentrations in theatmosphere are increasing, due at least in part to the burning of fossilfuels. The increasing level of carbon dioxide in the atmosphere is asignificant contributor to climate change.

The process of mining a diamond typically requires the burning of largequantities of fossil fuels and therefore the release of large volumes ofcarbon dioxide into the atmosphere. In particular, fossil-fuel-poweredvehicles are used to transport the rock in which diamonds are found in amine to the surface and then to a factory for processing. Very littlediamond by weight or volume is found in the rock by comparison to theoverall weight or volume of the rock, and the diamond that is yieldedfrom the rock must be further cut and polished to produce a gem. Thus,the vehicles must carry a significant amount of weight and volume ofrock to the factory in comparison to the finished gem which is yieldedafter processing at the factory. The processing of the rock at thefactory is also typically energy-intensive, since the rock has to becrushed to release the diamonds. The burning of fossil fuels in mining,transporting and processing diamonds therefore releases an appreciablevolume of carbon dioxide into the atmosphere. For example, the Ekatimine in Canada releases 65 kg (143 lb) of carbon dioxide per carat. (BHPBilliton 2008: Ekati mine emissions 195,179 metric tons of CO₂equivalent, production 3,000,000 carats of rough diamonds.)

An object of one or more embodiments is to address one or more of theseproblems.

SUMMARY

According to a first aspect of the invention, there is provided a methodof producing a synthetic diamond, the method comprising:

-   -   a) capturing carbon dioxide from the atmosphere;    -   b) conducting electrolysis of water to provide hydrogen;    -   c) reacting the carbon dioxide obtained from step (a) with the        hydrogen obtained from step (b) to produce methane; and    -   d) using the hydrogen obtained from step (b) and the methane        obtained from step c) to produce a synthetic diamond by chemical        vapour deposition.

In some aspects, the method of producing a synthetic diamond comprises:

-   -   (a) capturing carbon dioxide from the atmosphere;    -   (b) performing electrolysis of water to provide hydrogen;    -   (c-1) electrochemically reacting the carbon dioxide obtained        from step (a) with water to produce methane; and    -   (d) using the hydrogen obtained from step (b) and the methane        obtained from step (c-1) to produce a synthetic diamond by        chemical vapour deposition (CVD).

In accordance with certain aspects, step (a) may comprise capturingcarbon dioxide using a nanotube gas separator.

According to some aspects, step (a) may comprise capturing carbondioxide using a recyclable carbon dioxide sorbent or a polymer membranegas separator.

According to another aspect, step (a) may comprise capturing carbondioxide using an amine scrubber. Various amines are suitable for carbondioxide capture. According to a further aspect, the amine scrubber maybe monoethanolamine.

According to another aspect, step (a) may comprise capturing carbondioxide using an amine-based sorbent material. Various amine-basedsorbent materials are suitable for carbon dioxide capture. According toat least one aspect, capturing carbon dioxide using an amine-basedsorbent material may be performed at a temperature of 25° C. or below.According to some aspects, capturing carbon dioxide using an amine-basedsorbent material is performed at a temperature below 25° C.

According to some aspects, once carbon dioxide has been captured using asorbent material, the captured carbon dioxide is desorbed from thesorbent material by applying heat to the sorbent. For example, thecaptured carbon dioxide may be subsequently released by heating theamine-containing sorbent material to a temperature above 25° C.

According to another aspect, step (a) may comprise capturing carbondioxide using a mineral or a zeolite, which binds reversibly to carbondioxide. According to a further aspect, the mineral or zeolite maycomprise calcium oxide, serpentinite or molecular sieves.

According to certain aspects, step (a) may comprise capturing carbondioxide using a caustic solution. According to a further aspect, thecaustic solution may be calcium hydroxide solution, sodium hydroxidesolution, potassium hydroxide solution, lithium hydroxide solution orsoda lime.

In accordance with some aspects, step (a) may comprise capturing carbondioxide using activated carbon or lithium peroxide.

In accordance with another aspect, step (a) may comprise capturing air.According to certain aspects, the air captured may be ambient air. Inaccordance with a further aspect, the carbon dioxide is not capturedfrom flue gas.

According to various aspects, step (a) may comprise drying the air. Inaccordance with some aspects, a dessicant dryer may be used to dry theair. According to a further aspect, the air may be dried to a dew pointof less than −70° C. According to another aspect, the air may be driedto a dew point of substantially −70° C. In accordance with some aspects,step (a) may comprise cooling the air to liquefy carbon dioxide in theair. According to a further aspect, cooling the air to liquefy carbondioxide in the air may comprise compressing the air and then expandingthe air. According to a further aspect, the steps of compression andthen expansion of the air may be repeated. In accordance with a furtheraspect, the steps may be repeated three times. According to an evenfurther aspect, the steps may be repeated four times.

According to various aspects, one or more features discussed above maybe combined. For instance, in some aspects, carbon dioxide is capturedusing an amine-containing sorbent material at a temperature below 25°C., and the captured carbon dioxide is desorbed by applying heat to thesorbent material.

According to one or more aspects, step (b) may comprise conductingelectrolysis of water using at least one solid oxide electrolysis cell(SOEC). According to a further aspect, the SOEC may be a solid oxidefuel cell that is arranged to use a solid oxide or ceramic electrolyteto produce oxygen and hydrogen gas. According to another aspect, when atleast one SOEC is used for the electrolysis of water in step (b), theelectrolysis may be conducted at a temperature of between 350 and 1000°C. According to a further aspect, the electrolysis may be conducted at atemperature of between 500 and 850° C.

In accordance with some aspects, step (b) may comprise conductingelectrolysis of water using at least one polymer electrolyte membrane(PEM) cell. According to a further aspect, the PEM cell may be arrangedto use a solid polymer electrolyte (SPE) to produce hydrogen. Accordingto various aspects, when at least one PEM cell is used for theelectrolysis of water in step (b), the electrolysis may be conducted ata temperature below 150° C. According to a further aspect, theelectrolysis may be conducted at a temperature of 100° C. or below.According to an even further aspect, the electrolysis may be conductedat a temperature between 50 and 100° C.

According to certain aspects, step (b) may comprise conductingelectrolysis of water using at least one alkaline electrolysis cell(AEC). According to a further aspect, the alkaline electrolysis cell(AEC) may comprise two electrodes arranged to operate in a liquidalkaline electrolyte solution. According to a further aspect, the liquidalkaline electrolyte solution may comprise potassium hydroxide, sodiumhydroxide or potassium carbonate. In accordance with one or moreaspects, when at least one AEC is used for the electrolysis of water instep (b), the electrolysis may be conducted at a temperature of between50° C. and 300° C. According to a further aspect, the electrolysis maybe conducted at a temperature between 150° C. and 250° C.

According to at least one aspect, the water may comprise rainwater,potable water, water recycled from elsewhere in the diamond-productionprocess, and any combination of these. According to another aspect, step(b) may comprise filtering the water. According to another aspect, step(b) may comprise conducting reverse osmosis on the water. According to afurther aspect, the reverse osmosis may be conducted so as tosubstantially purify the water. According to various aspects, step (b)may comprise distilling the water. According to other aspects, step (b)may comprise de-ionising the water.

The majority of hydrogen produced globally is from the cracking ofmethane. This is not an environmentally friendly way to generatehydrogen as the methane comes from natural gas sources. Electrolysis ofwater is the most environmentally-friendly method of producing hydrogen,particularly when the energy used for this process is renewable energy(as discussed further below).

At least one benefit of the processes disclosed herein is that a numberof methane-production (methanation) methods may be used in step (c).These methane-production methods may, for instance, includethermochemical processes and biological processes and combinations ofthese processes. According to certain aspects, step (c) can be employedin conjunction with electrochemical processes for producing methane fromwater and carbon dioxide, which is discussed in relation to step (c-1)below.

In accordance with one or more aspects, step (c) may comprise reactingthe carbon dioxide with the hydrogen according to the Sabatier reaction;in other words, according to the Sabatier process. The Sabatier processis one example of a thermochemical process and involves the reaction ofhydrogen with carbon dioxide to produce methane and water, which isdescribed by the following exothermic reaction:CO₂+4H₂→CH₄+2H₂O+energy

According to at least one aspect, the reaction of step (c) may becarried out at a temperature between 100° C. and 800° C. According to afurther aspect, the reaction may be carried out at a temperature between150° C. and 600° C. According to a further aspect, the reaction may becarried out at a temperature between 250° C. and 450° C. According toanother aspect, the reaction may be carried out at a temperature ofsubstantially 250° C.

According to some aspects, the reaction of step (c) may be carried outat atmospheric pressure. According to other aspects, the reaction ofstep (c) may be carried out at a pressure above atmospheric pressure.According to a further aspect, the reaction of step (c) may be carriedout at a pressure of 0.7 barg.

According to at least one aspect, at least a portion of the waterproduced from the Sabatier process may be recycled or otherwise used inthe electrolysis process of step (b). In certain instances at least aportion of the water produced from the Sabatier process may be usedalone or in combination with rainwater or any other source of water inthe electrolysis process. This allows for an additional savings ofresources.

According to certain aspects, the reaction of step (c) may be carriedout in the presence of a catalyst. According to a further aspect, thecatalyst may be a nickel-based catalyst, a ruthenium-based catalyst, arhenium-based catalyst, a rhodium-based catalyst, or a cobalt-basedcatalyst. According to another aspect, the catalyst may comprise amixture of a nickel-based catalyst and a ruthenium-based catalyst. Inaccordance with various aspects, the catalyst may be supported orunsupported. According to a further aspect, the catalyst may besupported on a support comprising an oxide. According to another aspect,the catalyst may be supported on a support comprising TiO₂, SiO₂, MgO orAl₂O₃.

According to various aspects, the reaction of step (c) may be abiological process. For example, step (c) can be performed usingmethanogens. Methanogens are microorganisms that produce methane as ametabolic by-product in anoxic conditions. In some aspects, themethanogens are archaea, which are less sensitive to the environmentthey operate in. In accordance with some aspects, when the reaction ofstep (c) is performed using methanogens, the reaction may comprisereacting the carbon dioxide with the hydrogen according to the followingreaction:CO₂+4H₂→CH₄+2H₂O+energy

In other words, the reaction may be substantially the same as theSabatier process, but may be a natural reaction. In accordance with afurther aspect, the methanogens may be archaea or other prokaryotes.Step (c) may be carried out using conceivably any species of methanogen.In accordance with various aspects, the methanogens create methane aspart of their respiration process, where carbon dioxide is convertedinto methane under conditions that are generally low temperature (˜50°C.) and low pressure (atmospheric pressure or slightly above). Accordingto various aspects, step (c) may comprise purifying the methaneproduced.

According to one or more aspects, step (c) can be conducted as step(c-1), which comprises the electrochemical formation of methane fromcarbon dioxide, captured in step (a), and water. Step (c-1) has theadvantage that both input materials, carbon dioxide and water, areabundant natural resources. The electrochemical reaction below shows thereactants and a range of possible products that may be obtained when anelectric current is applied to conduct such a reaction:CO₂+H₂O+electric energy→CH₄+H₂+CO+C₂H₄

According to at least one aspect, the reaction of step (c-1) may becarried out at a temperature between 0° C. and 100° C. According to afurther aspect, the reaction may be carried out at a temperature between10° C. and 70° C. According to a further aspect, the reaction may becarried out at a temperature between 15° C. and 40° C. According toanother aspect, the reaction may be carried out at a temperature ofabout 25° C.

According to at least one aspect, the reaction of step (c-1) may becarried out at a pressure that is above atmospheric pressure. Accordingto a further aspect, the reaction may be carried out at a pressure offrom 1 to 10 atm. According to an alternative aspect, the reaction maybe carried out at a pressure below atmospheric pressure. According toanother aspect, the reaction may be carried out at atmospheric pressure.

According to at least one aspect, the reaction of step (c-1) is carriedout by providing electric energy in the form of direct current.According to a further aspect, the voltage of the direct current may be1.0-5.0 V. According to another aspect, the voltage of the directcurrent may be 1.5-2.5 V. According to another aspect, the voltage ofthe direct current may be 1.8-2.2 V.

According to at least one aspect, the reaction of step (c-1) is carriedout in the gaseous phase. According to a further aspect, the gas flowrate through the reactor is from 20 to 50 sccm.

According to at least one aspect, the reaction of step (c-1) is carriedout in the presence of a catalyst. According to certain aspects, thecatalyst is a metal catalyst. According to a further aspect, thecatalyst is a nickel-based catalyst. According to a further aspect, themetal catalyst is supported on a support material.

According to at least one aspect, the water may comprise rainwater,potable water, water recycled from elsewhere in the diamond-productionprocess, and combinations of these.

As mentioned above, step (c) can also be conducted using a combinationof one or more thermochemical processes and biological processes.According to another aspect, one or more thermochemical processes andbiological processes of step (c) can be carried out in combination withthe electrochemical process of step (c-1) in order to produce methane bydifferent reactions. In instances where one or more processes are usedin combination, the processes can, for instance, be used in series, inparallel, or a mixture of series and parallel.

According to various aspects, the methane produced by step (c) or step(c-1) may be purified using a pressure swing adsorption technique. Inaccordance with some aspects, step (c) or step (c-1) may comprise usingmolecular sieves to remove oxygen. According to certain aspects, step(c) or step (c-1) may comprise using zeolites to remove at least one ofcarbon dioxide, nitrogen, and water. In a similar manner as discussedabove with reference to the Sabatier process, according to at least oneaspect at least a portion of the water produced as a result of themethanation process using the methanogens may be recycled or otherwiseused in the electrolysis process of step (b).

In accordance with certain aspects, the methane may be purified usingcryogenics. According to one aspect, step (c) or step (c-1) may compriseusing membranes to remove nitrogen and CO₂. According to another aspect,step (c) or step (c-1) may comprise using desiccant drying to removewater.

According to some aspects, step (c) or step (c-1) may comprise using anamine gas treater to remove sulphide impurities.

Chemical vapour deposition (CVD) allows films of synthetic diamond to begrown over large areas of substrate with control over the properties ofthe diamond produced.

In accordance with one or more aspects, the CVD of step (d) may becarried out at a pressure of between 0.5 kPa and 100 kPa. According to afurther aspect the CVD step may be carried out at a pressure of between1 kPa and 50 kPa. According to an even further aspect, the CVD step maybe carried out at a pressure of substantially 40 kPa.

According to various aspects, the CVD of step (d) may be carried out ata temperature between 600° C. and 1200° C. According to a furtheraspect, the CVD step may be carried out at a temperature between 700° C.and 1000° C. According to another aspect, the CVD step may be carriedout at a temperature of substantially 950° C. According to certainaspects, the ratio of hydrogen to methane used in step (d) may be about5:1. According to other aspects, the ratio of hydrogen to methane usedin step (d) can be from 1:1 to 9:1, from 1:1 to 8:1, or from 5:1 to 8:1.

According to at least one aspect, the CVD of step (d) may be carried outon a substrate. In accordance with certain aspects, the substrate maycomprise diamond, silicon, tungsten, molybdenum, silicon carbide,silicon nitride, quartz glass or cemented carbide.

In accordance with one or more aspects, the CVD of step (d) may becarried out using a microwave plasma CVD machine.

According to certain aspects, step (a) may be performed before step (b).According to other aspects, step (b) may be performed before step (a) orsubstantially simultaneously with step (a).

In accordance with some aspects, the method may comprise an additionalstep, after step (d), of annealing the synthetic diamond. According to afurther aspect, the step of annealing the synthetic diamond may becarried out in a High Pressure High Temperature (HPHT) press.

In accordance with at least one aspect, the method may comprise a step,after step (d), and, in embodiments in which the synthetic diamond isannealed, after the step of annealing the synthetic diamond, of cuttingand/or polishing the diamond produced in step (d) to produce agem-quality diamond.

According to another aspect, the method may comprise the step ofreacting oxygen produced from the electrolysis of the water in step (b)with carbon produced by the chemical vapour deposition in step (d) toproduce carbon dioxide. According to a further aspect, the method maycomprise reacting the carbon dioxide obtained from this step with thehydrogen obtained from step (b) to produce methane. According to anotheraspect, the method may comprise adding the carbon dioxide to the carbondioxide captured from the atmosphere prior to step (c).

In accordance with certain aspects, the method may comprise separatingwater produced from the reaction of step (c) from the methane producedby that reaction. According to a further aspect, the separation of thewater from the methane may be conducted by centrifugation, condensationor adsorption. According to another aspect, the method may compriseelectrolysing the water to provide hydrogen.

According to at least one aspect, the method may comprise using energyfrom at least one renewable source to perform at least one of the steps.According to a further aspect, the method may comprise using energy fromat least one renewable source to perform the electrolysis. According toanother aspect, the method may comprise using energy from at least onerenewable source to perform each of the steps. As used herein, the term“renewable source” when used in reference to energy sources generallyrefers to one or more technologies that utilize replenishable energysources such as energy from water, wind, and the sun. For example,according to various aspects, the at least one renewable source maycomprise wind power. According to another aspect, the at least onerenewable source may comprise solar power. According to yet anotheraspect, the at least one renewable source may comprise wave power ortide power.

In accordance with embodiments in which the diamond produced in step (b)is cut and/or polished to produce a gem-quality diamond, in which energyfrom at least one renewable source is used to perform each of the steps,and in which the water in step (b) is rainwater, up to 230 times lesscarbon dioxide is released in the production of a gem-quality diamondthan the production of such a diamond from mining. According to certainaspects, approximately 40 times less carbon dioxide is released than inthe production of a gem-quality diamond using the process which releasesthe next-least quantity of carbon dioxide. It is estimated that if theglobal demand for diamonds (124 million carats in 2011) were switched todiamonds produced using this method, carbon dioxide emissions would bereduced by nearly 8 million tonnes per year.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments will now be described by way of example only, andwith reference to the accompanying drawings in which:

FIG. 1 is a flow diagram of an embodiment of the method of producing asynthetic diamond; and

FIG. 2 shows, schematically, an apparatus for use with the method.

DETAILED DESCRIPTION

Overview

With reference to FIG. 1, an example of the method for manufacturingdiamonds will now be described in overview.

First, carbon dioxide 2 is captured from the atmosphere; that is, carbondioxide 2 is captured from air 1. To capture carbon dioxide 2, thefollowing process can be used. Air 1, which has carbon dioxide 2 in it,is compressed 3 and then dried 4. Carbon dioxide 2 is then separated 5from the air 1 (although some air 1 will remain) and the separatedcarbon dioxide 2 and remaining air 1 are compressed and expanded to cool6 the carbon dioxide 2. The carbon dioxide 2 and remaining air 1 arecompressed and expanded 7, such that the carbon dioxide 2 liquefies. Theremaining air 1 is evacuated.

Second, water is electrolysed to provide hydrogen 11. In one embodiment,the water is rainwater 8. To produce hydrogen 11 from the rainwater 8,the following process is used. Rainwater 8 is captured, filtered,distilled and de-ionised 9. The resulting filtered and de-ionisedrainwater 8 is then electrolysed 10 to produce hydrogen 11 and oxygen12.

Third, carbon dioxide 2 obtained from the first step is reacted with thehydrogen 11 obtained from the second step to produce methane 14(methanation 13). The methanation 13 process is as follows. The hydrogen11 and carbon dioxide 2 are reacted according to the Sabatier reaction,described by the following equation:CO₂+4H₂→CH₄+2H₂O+energy

The products are methane 14 and water 8. As noted throughout thisapplication, the reaction of carbon dioxide with hydrogen to formmethane can also be conducted using a biological process, which caninvolve organisms known as methanogens. In further aspects, methanationcan be conducted by carried out by reacting carbon dioxide with water toproduce methane. In reference to FIG. 1, methanation 13 may be carriedout by reacting carbon dioxide 2 with water 8 rather than processingwater 8 through an electrolysis process 10 to produce hydrogen 11.

Fourth, the hydrogen 11 obtained from the second step and the methane 14obtained from the third step are used to produce a synthetic diamond 16using CVD 15.

Direct Air Capture of Carbon Dioxide

Referring to FIGS. 1 and 2, the process of capturing carbon dioxide 2 asdescribed above in overview above will now be described in more detail.An example of an apparatus used for this process will also be described.

The step of producing a synthetic diamond 16 using CVD requireshigh-purity carbon dioxide 2. In one embodiment, the purity required is99.9999%. The concentration of carbon dioxide in the atmosphere isaround 400 ppm. The process of capturing carbon dioxide 2 from the air 1therefore requires the concentration of carbon dioxide 2 to beincreased. Air 1, with carbon dioxide 2 in it, is drawn from a columnQ-1 into a compressor C-1. The air 1 and carbon dioxide 2 are compressed3 in the compressor C-1 (shown in FIG. 2). In one example, thecompressor C-1 is a high purity oil-free air compressor. The compressorC-1 increases the pressure of the air 1 and carbon dioxide 2 to a gaugepressure of around 12 barg.

Next, the air 1 and carbon dioxide 2 are dried 3. A desiccant dryer F-1can be used. In this embodiment, the air 1 and carbon dioxide 2 aredried 3 to a dew point of −70° C.

Carbon dioxide 2 is then separated 5 from the air 1 (although some air 1will remain), for example, using a nanotube gas separator. In thisembodiment, the nanotube gas separator is a nanotube 13X zeoliteadsorption stripper F-2. In this embodiment, the nanotube 13X zeoliteadsorption stripper F-2 strips out the carbon dioxide 2 from at least80% of the other air 1 components. The resulting gas stream is atambient pressure and rich in carbon dioxide 2.

The gas stream of air 1 and carbon dioxide 2 is then passed through aseries of turbo compressor-expanders C-2, C-3, C-4. Each turbocompressor-expander C-2, C-3, C-4 compresses the air 1 and carbondioxide 2 to condense the carbon dioxide 2 and then expands the air 1and carbon dioxide 2 to chill 6 them in a respective heat exchanger E-1,E-2, E-3. In this embodiment, there are three turbo compressor/expandersC-2, C-3, C-4, each with a heat exchanger E-1, E-2, E-3 through whichthe stream of air 1 and carbon dioxide 2 flows before it is againcompressed and expanded. However, it is to be appreciated that more orfewer turbo compressor-expanders can be used. The turbocompressor-expanders C-2, C-3, C-4 are such as those used in automotivetechnology. According to various aspects, step (a) may comprise dryingthe air.

In reference to the turbo compressor-expander device discussed above, inaccordance with some aspects, capturing carbon dioxide from theatmosphere may comprise cooling the air to liquefy carbon dioxide in theair. For example, cooling the air to liquefy carbon dioxide in the airmay comprise compressing the air and then expanding the air, such as byusing one or more of the turbo compressor-expander devices describedabove. According to a further aspect, the steps of compression and thenexpansion of the air may be repeated. In accordance with a furtheraspect, the steps may be repeated three times. According to an evenfurther aspect, the steps may be repeated four times.

Next, liquid/gas separation 7 is used to separate carbon dioxide 2 fromthe stream of air 1 and carbon dioxide 2. The carbon dioxide 2 andremaining air 1 are compressed in a compressor C-5. Then, they areexpanded in a tank V-1, such that the carbon dioxide 2 liquefies. Inthis embodiment, the carbon dioxide 2 is at a temperature of around −40°C. and a pressure of over 10 bar. The remaining air 1 is evacuated.

The liquid carbon dioxide 2 is warmed and returned into its gaseous formprior to being fed into a Sabatier reactor R-1 for methanation, asdescribed further below.

Carbon dioxide 2 can also be captured and prepared for methanation 13 byother means. For instance, carbon dioxide 2 can be captured on anamine-containing sorbent material at a temperature below 25° C. Forinstance, the sorbent material may be heated (to a temperature above 25°C.) to release the captured carbon dioxide gas. In the context of FIG.1, this would process carbon dioxide 2 to methanation 13. In the contextof FIG. 2, this would process from column Q-1 to tank V-1 followingdifferent processing steps than those outlined in FIG. 2 between columnQ-1 and tank V-1.

Hydrogen Generation

With continued reference to FIGS. 1 and 2, the second step ofelectrolysing water 8 to provide hydrogen 11, will now be described inmore detail. An example of the apparatus used in this step will also bedescribed.

Rainwater 8 is collected via a rainwater 8 funnel Q-2 and stored in atank 18. In a variation of this process, water may be collected in tank18 from a source other than rainwater, such as potable water, and/orwater recycled from elsewhere in the overall diamond-production process.The water 8 is transferred via a metering pump 19 connected to the tank18 into a strainer 20. The metering pump 19 operates at low pressure. Inthis embodiment, the pressure at which the metering pump 19 operates isabout 0.5 barg. The water 8 passes through the strainer 20 and isstrained by the strainer 20. Next, the water 8 passes through a filter21 which filters the water 8 relatively coarsely. From the filter 21,the water 8 enters a reverse osmosis purification unit 22, which furtherpurifies the water 8 using reverse osmosis. Next, the water 8 isdistilled in a still (not shown) and de-ionised in a de-ioniser 23. Inan alternative process, water 8 can be purified by reverse osmosis.

From the de-ioniser 23, the water passes to an electrolyser X-1. Theelectrolyser X-1 is powered by renewable energy. For example, theelectrolyser X-1 can be powered by wind, solar, wave, tide, orgeothermal energy. The electrolyser X-1 splits the water 8 into hydrogen11 and oxygen 12 via an electrolysis 10 process. In this embodiment, theresulting hydrogen 11 is at about 99.999% purity. The oxygen 12 is takenoff into a waste gas oxidiser R-2, to be used in a waste gas recoveryprocess, as will be described further below. The hydrogen 11 is takeninto a hydrogen storage tank T-1.

The hydrogen storage tank T-1 is a specialist lab equipment tank thatstores the hydrogen 11 input at 99.999% purity and returns the hydrogen11 at 99.9999% purity. The tank T-1 is a container containing solidmetal hydride. In this embodiment, the solid metal hydride is anAB5-type alloy. Pressure is regulated using let-down valves from thestored pressure, which in one example can be as high as 10 barg.

In accordance with some aspects, electrolysis of water may be performedusing at least one polymer electrolyte membrane (PEM) cell. According toa further aspect, the PEM cell may be arranged to use a solid polymerelectrolyte (SPE) to produce hydrogen. According to various aspects,when at least one PEM cell is used for the electrolysis of water, theelectrolysis may be conducted at a temperature below 150° C. Accordingto a further aspect, the electrolysis may be conducted at a temperatureof 100° C. or below. According to an even further aspect, theelectrolysis may be conducted at a temperature between 50 and 100° C.

Methanation

As discussed above, the third step of the present method for themanufacture of a synthetic diamond involves the reaction of the carbondioxide obtained from the first step with the hydrogen obtained from thesecond step to produce methane. This process, and the apparatus used forit, will now be described in more detail, with continued reference toFIGS. 1 and 2.

Carbon dioxide 2 obtained from the first step is stored, as describedabove, in a tank V-1. Hydrogen 11 from the second step is stored, asdescribed above, in a hydrogen storage tank T-1. The carbon dioxide 2and hydrogen 11 are drawn from these tanks V-1, T-1 into a Sabatierreactor R-1. In the Sabatier reactor R-1, the carbon dioxide 2 isreacted with the hydrogen 11 to produce methane 14 (methanation 13). Thehydrogen 11 and carbon dioxide 2 are reacted according to the Sabatierreaction, described by the following equation:CO₂+4H₂→CH₄+2H₂O+energy

According to certain aspects, the reaction may be carried out in thepresence of a catalyst. In one embodiment, a Ruthenium catalyst is used.The reaction takes place at 250° C. and around 0.7 barg. Under theseconditions, over 95% of the carbon dioxide 2 is converted, withsubstantially 100% selectivity of methane 14. That is, over 95% of thecarbon dioxide 2 is converted exclusively to methane, withoutappreciable conversion of carbon dioxide 2 to any other product.According to at least one aspect, the reaction of may be carried out ata temperature between 100° C. and 800° C. According to a further aspect,the reaction may be carried out at a temperature between 150° C. and600° C. According to a further aspect, the reaction may be carried outat a temperature between 250° C. and 450° C. According to anotheraspect, the reaction may be carried out at a temperature ofsubstantially 250° C.

According to other aspects, the catalyst may be a nickel-based catalyst,a rhenium-based catalyst, a rhodium-based catalyst, or a cobalt-basedcatalyst, or any combination thereof. According to another aspect, thecatalyst may comprise a mixture of a nickel-based catalyst and aruthenium-based catalyst. In accordance with various aspects, thecatalyst may be supported or unsupported. According to a further aspect,the catalyst may be supported on a support comprising an oxide.According to another aspect, the catalyst may be supported on a supportcomprising TiO₂, SiO₂, MgO or Al₂O₃.

According to various aspects, the methanation reaction may be performedusing methanogens. As discussed above, methanogens are microorganismsthat produce methane as a metabolic by-product in anoxic or anaerobicconditions. In accordance with some aspects, when methanation isperformed using methanogens, the reaction may comprise reacting thecarbon dioxide with the hydrogen according to the following reaction:CO₂+4H₂→CH₄+2H₂O+energy

In other words, the reaction may be substantially the same as theSabatier process, but may be a natural reaction. In accordance with afurther aspect, the methanogens may be archaea or other prokaryotes.Methanation may be carried out using conceivably any species ofmethanogen. In accordance with various aspects, the methanogens createmethane as part of their respiration process, where carbon dioxide isconverted into methane under conditions that are generally lowtemperature (−50° C.) and low pressure (atmospheric pressure or slightlyabove).

As explained above, according to various aspects, step (c-1) may includeproducing methane by the electrochemical reaction of water with carbondioxide, instead of, or in addition to, the reaction of carbon dioxidewith hydrogen obtained by the electrolysis of water. For example, inreference to FIGS. 1 and 2, electrochemical methanation is conductedusing carbon dioxide 2 that has been stored in tank V-1, and purifiedwater. The water can, for example, be purified in accordance with theprocesses described above (e.g., the subsection titled “HydrogenGeneration”) or by reverse osmosis.

According to various aspects, the purified water and carbon dioxide maybe fed to one or more electrochemical cells in the gaseous phase at aflow rate of 20-50 sccm. According to another aspect, theelectrochemical cell may be powered by direct current at 1.8-2.2 V andmay contain a catalyst comprising nickel.

Referring back to FIGS. 1 and 2, the methane 14 produced is thenpurified in a methane purification apparatus F-3. This apparatus F-3purifies the methane 14 by removing any impurities, including the water8 that is also a product of the Sabatier process or that is present asan unreacted input material for the electrochemical process. In thisembodiment, the water 8 is collected via centrifugation, althoughcondensation or an adsorption method can be used as alternatives. Inthis embodiment, the water 8 is added to (this is not shown) the water 8that is electrolysed 10 in the electrolyser X-1 to be used for thecreation of more hydrogen 11. The methane 14 is stored in a methane tankT-2, ready for use.

According to certain aspects, and within the context of theelectrochemical methanation process, hydrogen gas produced as aby-product may be fed to the CVD diamond manufacture step as aco-reagent. Hydrogen gas produced as a by-product of the electrochemicalmethanation process may also be fed to the thermochemical and/orbiological methanation steps, if present, in order to react with carbondioxide to produce methane.

According to various aspects, the methane may be purified using apressure swing adsorption technique. In accordance with some aspects,methanation may comprise using molecular sieves to remove oxygen.According to certain aspects, methanation may comprise using zeolites toremove at least one of carbon dioxide, nitrogen, and water. In a similarmanner as discussed above with reference to the Sabatier process,according to at least one aspect at least a portion of the waterproduced as a result of the methanation process using the methanogensmay be recycled or otherwise used in the electrolysis process.

In accordance with certain aspects, the methane may be purified usingcryogenics. According to one aspect, methanation may comprise usingmembranes to remove nitrogen and CO₂. According to another aspect,methanation may comprise using desiccant drying to remove water.According to some aspects, methanation may comprise using an amine gastreater to remove sulphide impurities.

CVD Diamond Manufacture

The fourth step of the present method for the manufacture of a syntheticdiamond 16 comprises utilising the hydrogen 11 obtained from the secondstep and the methane 14 obtained from the third step, to produce asynthetic diamond 16 using a CVD process 15. This CVD process 15 in thecontext of the present embodiment, and the apparatus that is used toconduct the CVD process 15 will now be described, with continuedreference to FIGS. 1 and 2.

In one embodiment, the aim is to produce gem quality diamonds suitablefor the jewellery market. The hydrogen 11 and the methane 14 are drawninto a CVD diamond machine CVD-1 from their respective tanks T-1, T-2.In one embodiment, the CVD diamond machine is a microwave plasma CVDmachine. The gas mixture fed into the CVD diamond machine CVD-1 is at aratio of about 5:1 hydrogen:methane. A microwave CVD process 15 is usedto make a gem-quality single crystal diamond 16. In this embodiment, theCVD takes place at a pressure of 40 kPa (300 Torr) and a temperature ofaround 950° C. A power source of less than 50 kW produces a plasma zoneon a diamond substrate. For example, the diamond substrate may comprisea diamond seed crystal that may then be used as the initial base fordiamond crystal growth. Carbon drops out onto the lattice of thesubstrate to grow a diamond 16 that is ready for finishing.

In one embodiment, negatively-charged nitrogen inclusions are used tocreate green-coloured diamonds. In other embodiments, other dopants maybe used to create other colours of diamonds.

In this embodiment, oxygen 12 is also purged from the CVD diamondmachine CVD-1 to stop the formation of soot. This oxygen 12, along withcarbon emissions from the machine, is sent to the waste gas oxidiserR-2. As discussed above, oxygen 12 from the electrolysis 10 process isalso sent to the waste gas oxidiser R-2. In the waste gas oxidiser R-2,oxidation 17 of the carbon occurs, producing further carbon dioxide 2.According to one example, this carbon dioxide is added to the column Q-1to increase the concentration of carbon dioxide 2 in the carbon dioxide2 capture process. Remaining gases are made safe before emission to theatmosphere. According to another example, the carbon dioxide is reactedwith the hydrogen obtained from the electrolysis process to producemethane.

In accordance with one or more aspects, the CVD process may be carriedout at a pressure of between 0.5 kPa and 100 kPa. According to a furtheraspect the CVD step may be carried out at a pressure of between 1 kPaand 50 kPa. According to an even further aspect, the CVD step may becarried out at a pressure of substantially 40 kPa.

According to various aspects, the CVD process may be carried out at atemperature between 600° C. and 1200° C. According to a further aspect,the CVD step may be carried out at a temperature between 700° C. and1000° C. According to another aspect, the CVD step may be carried out ata temperature of substantially 950° C. According to certain aspects, theratio of hydrogen to methane used in the CVD process may besubstantially 5:1.

According to at least one aspect, the CVD process may be carried out ona substrate. In accordance with certain aspects, the substrate maycomprise diamond, silicon, tungsten, molybdenum, silicon carbide,silicon nitride, quartz glass or cemented carbide.

In accordance with one or more aspects, the CVD process may be carriedout using a microwave plasma CVD machine.

Diamond Processing

In a final step (not shown) the diamond 16 is processed to gem qualityby cutting and polishing.

In accordance with some aspects, after CVD processing, the syntheticdiamond may be annealed. According to one example, the step of annealingthe synthetic diamond may be carried out in a High Pressure HighTemperature (HPHT) press.

In accordance with at least one aspect, the synthetic diamond may be cutand polished after the annealing step is performed to produce agem-quality diamond.

According to at least one aspect, one or more of the processes discussedherein may include using energy from at least one renewable source. Forexample, electrolysis may comprise using energy from at least onerenewable source. Thus, one or more technologies that utilizereplenishable energy sources such as energy from water, wind, and thesun may be used to perform at least one of the processes discussedherein (i.e., carbon dioxide capture, electrolysis, methanation, CVDdiamond formation). For example, according to various aspects, the atleast one renewable source may comprise wind power, solar power, wavepower, tide power, and/or geothermal power. According to a furtheraspect, renewable energy may be used to perform each of the processesdiscussed herein.

Having thus described several aspects of at least one example, it is tobe appreciated that various alterations, modifications, and improvementswill readily occur to those skilled in the art. For instance, examplesdisclosed herein may also be used in other contexts. Such alterations,modifications, and improvements are intended to be part of thisdisclosure, and are intended to be within the scope of the examplesdiscussed herein. Accordingly, the foregoing description and drawingsare by way of example only.

What is claimed is:
 1. A method of producing a synthetic diamond, themethod comprising: (a) capturing carbon dioxide from an atmosphere; (b)performing electrolysis of water to provide hydrogen; (c-1)electrochemically reacting the carbon dioxide obtained from step (a)with water to produce methane; and (d) using the hydrogen obtained fromstep (b) and the methane obtained from step (c-1) to produce thesynthetic diamond by chemical vapour deposition (CVD); and (e) reactingoxygen produced from the electrolysis of the water in step (b) withcarbon emissions produced by the chemical vapour deposition in step (d)to produce carbon dioxide.
 2. The method of claim 1, wherein step (a)comprises capturing carbon dioxide using a nanotube gas separator. 3.The method of claim 1, wherein step (a) comprises cooling air to liquefycarbon dioxide in the air.
 4. The method of claim 3, wherein cooling theair to liquefy carbon dioxide in the air comprises compressing the airand then expanding the air.
 5. The method of claim 4, wherein the stepsof compressing the air and then expanding the air are repeated.
 6. Themethod of claim 5, wherein the steps are repeated three times.
 7. Themethod of claim 1, wherein step (a) comprises capturing carbon dioxideusing an amine-containing sorbent-material.
 8. The method of claim 7,wherein capturing carbon dioxide using the amine-coated sorbent-materialis conducted at a temperature below 25° C.
 9. The method of claim 8,wherein the captured carbon dioxide is subsequently released by heatingthe amine-containing sorbent material to a temperature above 25° C. 10.The method of claim 1, wherein step (b) comprises performing theelectrolysis of water using at least one polymer electrolyte membrane(PEM) cell.
 11. The method of claim 10, wherein the electrolysis isperformed at a temperature below 150° C.
 12. The method of claim 1,wherein step (c-1) is conducted in the presence of a metal catalyst. 13.The method of claim 12, wherein the metal catalyst is a nickel-basedcatalyst.
 14. The method of claim 1, wherein step (c-1) is conductedwith a direct current.
 15. The method of claim 1, wherein the CVD iscarried out at a pressure of between 0.5 kPa and 100 kPa.
 16. The methodof claim 15, wherein the CVD is carried out at a pressure ofsubstantially 40 kPa.
 17. The method of claim 1, wherein the CVD iscarried out at a temperature between 700° C. and 1000° C.
 18. The methodof claim 17, wherein the CVD is carried out at a temperature ofsubstantially 950° C.
 19. The method of claim 1, wherein a ratio ofhydrogen to methane used in step (d) is in a range of from 1:1 to 9:1.20. The method of claim 19, wherein the ratio of hydrogen to methaneused in step (d) is about 5:1.
 21. The method of claim 1, wherein theCVD is carried out on a substrate comprising diamond.
 22. The method ofclaim 1, further comprising reacting the carbon dioxide obtained fromstep (e) with the hydrogen obtained from step (b) to produce methane.23. The method of claim 1, wherein the method further comprises usingenergy from at least one renewable source to perform at least one of thesteps (a)-(d).
 24. The method of claim 23, wherein the method furthercomprises using energy from at least one renewable source to perform theelectrolysis.
 25. The method of claim 1, wherein the water used in step(b) comprises rainwater.
 26. The method of claim 1, wherein the waterused in step (c-1) comprises rainwater.
 27. The method of claim 1,wherein the method further comprises an additional step, after step (d),of annealing the synthetic diamond.
 28. The method of claim 1, whereinthe method further comprises an additional step of at least one ofcutting and polishing the diamond produced in step (d).